The primary structure of a proteolipid component of the mitochondrial ATPase complex has been determined from wild type and mutant strains of Sacharomyces cerevisiae. Three well known genetic markers (oli 1, oli 3 and pho 2) on mitochondrial DNA have been shown to be localized within the structural gene of the proteolipid. Cytoplasmic petite mutants of yeast have been isolated whose retained segments of mitochondrial DNA contain the structural gene of the proteolipid. The DNA of one such mutant has been sequenced. The DNA sequence establishes that mitochondria utilize the codons of the universal code but that in yeast the code is highly non-degenerate. The sequencing studies have also revealed that mitochondrial DNA of yeast contain long stretches of A plus T sequences interspersed with short stretches of G plus C. The function of these non-structural regions is not known at present.